1. Field of the Invention
The present invention relates to a resistor for an electron gun assembly that is mounted in a cathode-ray tube, and more particularly to a resistor for an electron gun assembly, the resistor being configured to apply a voltage, which is divided with a predetermined resistance division ratio, to a grid electrode provided in the electron gun assembly, an electron gun assembly with the resistor for an electron gun assembly, and a cathode-ray tube with the electron gun assembly.
2. Description of the Related Art
In recent years, there is an increasing demand for the advent of a cathode-ray tube capable of displaying a high-resolution color image. A beam spot size that is a major factor for determining resolution is determined by the focusing performance of an electron gun assembly that is mounted in the cathode-ray tube. In general terms, the focusing performance is determined by an aperture of a main lens, a virtual object point size, a magnification, etc. In other words, as the aperture of the main lens increases, as the virtual object point size decreases and as the magnification decreases, the size of a beam spot that is formed on a phosphor screen can be reduced and the resolution can be increased.
The electron gun assembly that is required to have such a good focusing performance is provided with various grid electrodes, which are supplied with relatively high voltages, in addition to an anode that is supplied with an anode voltage. As regards the cathode-ray tube with this structure, a problem of withstand voltage arises if high voltages are applied to the respective grid electrodes from a stem section of the cathode-ray tube.
To solve the problem, a resistor for dividing a voltage (an electron gun assembly resistor) is incorporated along with the electron gun assembly in the cathode-ray tube. The electron gun assembly resistor divides an anode voltage with a predetermined resistance division ratio. Desired high voltages, which are divided by the electron gun assembly resistor, are applied to predetermined grid electrodes (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 09-017352).
The electron gun assembly resistor includes, on an insulating substrate, an electrode element formed of a low-resistance material, and a resistor element formed of a high-resistance material that is basically similar to the material of the electrode element. A part of the electrode element and the resistor element are coated with an insulating coating layer. A terminal portion that is formed of a metal terminal is electrically connected to the electrode element. The terminal portion is fixed by calking to a through-hole that is formed in the insulating substrate.
However, in some cases, there arise various problems with the cathode-ray tube in which the above-described resistor is disposed.
For example, in order to improve withstand voltage characteristics, the cathode-ray tube, to which the above-mentioned high voltages are applied, is subjected to a withstand voltage process after an evacuation process in the fabrication steps. In the withstand-voltage process, a high voltage, which has a peak voltage about twice or thrice as high as a normal operation voltage, is applied to the cathode-ray tube. This causes a forcible discharge and removes burr or attached matter from the various grid electrodes, which may lead to deterioration in withstand-voltage characteristics.
A surface creepage, which occurs when the withstand voltage process is performed, progresses along the surface of the insulating coating layer of the resistor. Consequently, a discharge current may flow to a resistor element or an electrode element that lies under the insulating coating layer, leading to dielectric breakdown. Further, at the same time as the dielectric breakdown, the insulating coating layer that is in contact with the electrode element may be damaged. Moreover, matter that has peeled off the resistor and dropped floats within the cathode-ray tube and may clog the apertures of the shadow masks. In some cases, the resistor element, which is connected to the electrode element, may be damaged and, at last, line breakage may occur in the resistor element.
Such problems may be solved to some extent by relaxing conditions for the withstand voltage process, or properly controlling conditions for the withstand voltage process. However, a problem of degradation in focusing performance due to glow discharge, which is to be described below, is a very serious one for the cathode-ray tube that is required to have a high resolution.
To be more specific, while the cathode-ray tube is in operation, a glow discharge may occur, which originates from an edge of an electrode element that adjoins a ceramic insulating substrate, or from an exposed ceramic portion, and extends toward the high-voltage side. Such a glow discharge supplies an unnecessary current into the resistor. In other words, an excess current flows to the grid electrode, which is supplied with a voltage via the resistor, and a voltage, which is divided at a predetermined resistance division ratio, cannot stably be supplied. Consequently, such a phenomenon causes a focusing defect of an electron beam that is focused on the phosphor screen, and degrades the quality of an image that is displayed on the cathode-ray tube.
It may be thought that such a phenomenon of glow discharge occurs due to charge-up of an exposed ceramic part with a high secondary-electron emission coefficient. It is thus proposed that the exposed ceramic part is coated with an insulating coating layer, thereby suppressing occurrence of glow discharge.
However, if the exposed ceramic part is coated with the insulating coating layer, the above-mentioned dielectric breakdown due to the discharge current at the time of the withstand voltage process may easily occur at, or near, an overlapping part where the coated insulating coating layer contacts the electrode element. As a result, peeling of the insulating coating layer occurs, and such a defect as clogging of holes in the shadow mask may occur.